1.Field of the Invention
This invention relates to a method and an apparatus for controlling the movement of an object with respect to a reference and, more specifically, to a method and an apparatus for moving exposure masks and/or substrate wafers for use in fabricating micro-lithographically generated circuits.
2.Description of the Prior Art
In general, the known lithographic techniques for fabricating microcircuits requires the accurate alignment of the substrate wafer with respect to the exposure mask. In the past, simple mechanical, screw-type, means for aligning the mask and the wafer have been used to align the translational and rotational positions of the mask and wafer. In this type of system, however, inevitable tolerances and torques build-up which affect the alignment.
In addition, other mechanical techniques for aligning the masks and wafers have been attempted, however, an electromechanical technique has been proposed to offset some of the problems of the prior art mechanical techniques. This electromechanical method for aligning the mask and substrate wafers envisions using a piezoelectric transducer coupled to the mask or wafer to displace the mask or wafer with respect to the other in response to a voltage applied to the transducer. For relatively low voltages, it is well known that the displacement of the transducer varies directly with the applied voltage. The displacement of a mask or wafer coupled to a transducer, therefore, is substantially linear with voltage such that precise electromechanical control, movement, of the wafer may be exercised by means of precise voltage control. At relatively high voltages, however, the linearity of transducer displacement with respect to voltage breakdown, thus the operating range of the transducer is limited by its breakdown voltage, which limits the total displacement of the wafer. That is, only limited displacements within the range of linearity of the transducer are available by means of this technique.
Accordingly, means for accurately effecting movement of an object such as an exposure mask or a substrate wafer relative to a reference or each other is required which is capable of effecting movement of the object beyond the range of linear motion of the transducer.